Rare-earth doped optical materials have been studied extensively over the past 40 years. The unique electronic structure of the lanthanides, comprised of partially filled 4f orbitals, lends itself well to photonics. The energies associated with these intra 4f electron transitions are located in the ultraviolet (UV), visible (VIS), and infrared (IR). Researchers have therefore employed these materials in the production and manipulation of light at these wavelengths. A significant portion of the research in this area has focused on the trivalent erbium ion. This is largely due to the 4I15/2→4I13/2 transition of Er3+ at 1.5 μm which coincides with the minimum loss wavelength of silica optical fibers. One of the most important innovations to emerge from this work is the Erbium-doped fiber amplifier (EDFA) which has revolutionized telecommunications. The ability to optically amplify signals in an integrated fiber amplifier has led to the realization of long-haul optical fiber networks.
Recently, research has been directed toward increasing the bandwidth in optical fibers, which entails increasing the spectral width of the erbium emission (see Table 1 shown below). Wavelength division multiplexing (WDM) allows many signals to be sent down an optical fiber at once, each comprising its own channel. In addition, there has been a great deal of work in “flattening” the fluorescence emission to obtain equal gain across a broad range of wavelengths, thus increasing the number of channels. Conventional research has examined tailoring the composition to promote broader and flatter emissions. Some of the most promising results were obtained with alumino-silicate glasses, but a material with a true flat emission has yet to be uncovered. The integration of nanotechnology and photonic materials, often dubbed nanophotonics, offers a route to develop such a material. The quantum confinement effects associated with small nanoparticles adds a new method for achieving novel optical properties by introducing the concept of tunability. Recent studies have shown size effects on the optical properties of various materials which opens the door for tunable photonic materials. By manipulating the nanostructure of the material, its fluorescence emission characteristics can be modified.
TABLE 1Optical parameters of Er3+ in a range of host materialsLumines-Peak-cencePeakPeakstimulatedbandwidthMaximumabsorptionabsorptionemission(1535 nmopticallycross-sectioncross-section4I13/2 → 4I15/2Upconversioncross-sectionFWHM atactive(488 nm)(980 nm)PL lifetimeacoefficient(1535 nm)300 K)concentrationSilica<8.0 × 10−21 cm2 1.0 × 10−21cm212 × 10−3s3.0 × 10−21cm27.27 × 10−21cm211 nm0.1 at %(melt glass)(PECVD)Phospho-2.01 × 10−21cm210 × 10−3s9.0 × 10−21cm3 s−127 nm2.5 at %silicateglassAlumino-3.12 × 10−21cm210 × 10−3s1.0 × 10−16cm3 s−15.7 × 10−21cm243 nm500 ppmsilicateglassSilicon2-8 × 10−12 cm2 420 × 10−6s3 × 1017 cm−3(crystal-(514 nm)line)Amorphous1.4 × 10−14 cm2800 × 10−6ssilicon(514 nm)Silicon-7.3 × 10−17 cm2—~2.5 × 10−3sUp to 60 nm—rich(depends on(depends onsilicaSi content)Si content)Porous1 × 10−3s−10 nm siliconAlumina2.0 × 10−21cm−27.8 × 10−3s4.0 × 10−18cm3 s−16.0 × 10−21cm255 nmGaN4.8 × 10−21cm−22.95 × 10−3s~8 nmGaAs1 × 10−3s1 × 10−3cm27 × 1017 cm−3ZBLAN5.0 × 10−21cm218 mol %Lithium3.0 × 10−3s<1.4 × 10−19cm3 s−1niobateYAG5.4 × 10−17cm3 s−1PPMA1.1 × 10−20 cm20.8 × 10−6s70 nmTellu-4.48 × 10−21cm−23.3 × 10−3s2.74 × 10−17cm3 s−11.3 × 10−20cm−280 nm2.5 at %riteaMaximum value reported in unclustered material.